Gain control metric computation in a wireless repeater

ABSTRACT

A method for computing a gain control metric used in controlling gain in a wireless repeater operates to store correlation and normalization values associated with the gain control metric for the previous N samples in registers. For each new sample of the gain control input signal, the correlation and normalization values are computed by discarding the multiplication terms of the obsolete sample and adding the multiplication terms of the new sample to the stored correlation and normalization values. In this manner, the complexity of the computation is greatly reduced and the complexity of the computation does not increase with the integration length.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/177,209, filed on May 11, 2009, whichapplication is incorporated herein by reference in its entirety.

This application is related to the following concurrently filed andcommonly assigned U.S. patent applications: Application serial no.xx/xxx,xxx, entitled “Gain Control Optimizing SINR and Data Rate ForWireless Repeater”; Application serial no. xx/xxx,xxx, entitled“Multi-Metric Gain Control For Wireless Repeater”; Application serialno. xx/xxx,xxx, entitled “Stability Indicator For A Wireless Repeater”;Application serial no. xx/xxx,xxx, entitled “Gain Adjustment SteppingControl In A Wireless Repeater”; and Application serial no. xx/xxx,xxx,entitled “Gain Control Metric Pruning In A Wireless Repeater”. Theapplications are incorporated herein by reference in their entireties.

BACKGROUND

1. Field

This disclosure generally relates to repeaters in wireless communicationsystems.

2. Background

Wireless communication systems and techniques have become an importantpart of the way we communicate. However, providing coverage can be asignificant challenge to wireless service providers. One way to extendcoverage is to deploy repeaters.

In general, a repeater is a device that receives a signal, amplifies thesignal, and transmits the amplified signal. FIG. 1 shows a basic diagramof a repeater 110, in the context of a cellular telephone system.Repeater 110 includes a donor antenna 115 as an example networkinterface to network infrastructure such as a base station 125. Repeater110 also includes a server antenna 120 (also referred to as a “coverageantenna”) as a mobile interface to mobile device 130. In operation,donor antenna 115 is in communication with base station 125, whileserver antenna 120 is in communication with mobile devices 130.

In repeater 110, signals from base station 125 are amplified usingforward link circuitry 135, while signals from mobile device 130 areamplified using reverse link circuitry 140. Many configurations may beused for forward link circuitry 135 and reverse link circuitry 140.

There are many types of repeaters. In some repeaters, both the networkand mobile interfaces are wireless; while in others, a wired networkinterface is used. Some repeaters receive signals with a first carrierfrequency and transmit amplified signals with a second different carrierfrequency, while others receive and transmit signals using the samecarrier frequency. For “same frequency” repeaters, one particularchallenge is managing the feedback that occurs since some of thetransmitted signal can leak back to the receive circuitry and beamplified and transmitted again.

Existing repeaters manage feedback using a number of techniques; forexample, the repeater is configured to provide physical isolationbetween the two antennae, filters are used, or other techniques may beemployed.

SUMMARY

Systems, apparatuses, and methods disclosed herein allow for enhancedrepeater capability. In one embodiment, a method for computing a gaincontrol metric used in controlling gain in a wireless repeater isdescribed. The wireless repeater receives an input signal at a receivingantenna and transmits an output signal on the transmitting antenna wherethe input signal is a sum of a remote signal to be repeated and afeedback signal resulting from a feedback channel between the receivingantenna and a transmitting antenna, the feedback channel thus forming afeedback loop between the transmitting antenna and the receivingantenna. The method includes providing a tap-specific gain controlmetric indicative of a loop gain of the repeater where the tap-specificgain control metric at a given channel tap is computed from a gaincontrol input signal and is a square of a correlation term divided by anormalization term, the correlation term and the normalization term bothhaving an integration length of N. The gain control input signal can betaken from any point in the feedback loop of the repeater. In oneembodiment, the correlation term is indicative of a correlation of thegain control input signal and a delayed version of the gain controlinput signal and the normalization term is indicative of a power of thegain control input signal that is not delayed. The method furtherincludes providing a first register for storing a stored correlationvalue where the stored correlation value is the correlation termcomputed over a previous N samples, providing a second register forstoring a stored normalization value where the stored normalizationvalue is the normalization term computed over a previous N samples, andreceiving a new sample of the gain control input signal. The methodfurther includes computing an updated correlation value by performing afirst multiplication using an obsolete sample of the gain control inputsignal where the obsolete sample is the (N+1)th previous sample, and asecond multiplication using the new sample of the gain control inputsignal. The updated correlation value is computed by adding the productfrom the second multiplication and subtracting the product from thefirst multiplication. The method further includes computing an updatednormalization value by performing a third multiplication using anobsolete sample of the gain control input signal where the obsoletesample is the (N+1)th previous sample, and a fourth multiplication usingthe new sample of the gain control input signal. The updatednormalization value is computed by adding the product from the fourthmultiplication and subtracting the product from the thirdmultiplication. Finally, the method includes computing the tap-specificgain control metric using the updated correlation value and the updatednormalization value.

According to another embodiment of the present invention, a wirelessrepeater having a receiving antenna for receiving an input signal and atransmitting antenna for transmitting an output signal is described. Theinput signal is a sum of a remote signal to be repeated and a feedbacksignal resulting from a feedback channel between the receiving antennaand the transmitting antenna and the feedback channel thus forms afeedback loop between the transmitting antenna and the receivingantenna. The wireless repeater includes a gain control block configuredto control a gain of the repeater. The gain control block includes again metric calculator configured to receive a gain control input signaland to compute a tap-specific gain control metric indicative of a loopgain of the repeater. The tap-specific gain control metric at a givenchannel tap is a square of a correlation term divided by a normalizationterm where the correlation term and the normalization term have anintegration length of N and the correlation term is indicative of acorrelation of the gain control input signal and a delayed version ofthe gain control input signal and the normalization term is indicativeof a power of the gain control input signal that is not delayed. Thegain metric calculator includes a first register configured to store astored correlation value where the stored correlation value is thecorrelation term computed over a previous N samples of the gain controlinput signal and a second register configured to store a storednormalization value where the stored normalization value is thenormalization term computed over a previous N samples of the gaincontrol input signal. In operation, the gain metric calculator block isconfigured to receive a new sample of the gain control input signal andthe gain metric calculator is configured to compute an updatedcorrelation value by performing a first multiplication using an obsoletesample of the gain control input signal, the obsolete sample being the(N+1)th previous sample, and a second multiplication using the newsample of the gain control input signal, the updated correlation valuebeing computed by adding the product from the second multiplication andsubtracting the product from the first multiplication. Furthermore, thegain metric calculator is configured to compute an updated normalizationvalue by performing a third multiplication using an obsolete sample ofthe gain control input signal, the obsolete sample being the (N+1)thprevious sample, and a fourth multiplication using the new sample of thegain control input signal, the updated normalization value beingcomputed by adding the product from the fourth multiplication andsubtracting the product from the third multiplication. The gain metriccalculator is configured to compute the tap-specific gain control metricusing the updated correlation value and the updated normalization value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a repeater according to the prior art.

FIG. 2 shows a diagram of a repeater environment according to someembodiments of the current disclosure.

FIG. 3 illustrates the effect of feedback signal on the noise floor atthe repeater input.

FIG. 4 illustrates the effect of feedback signal on the noise floor atthe repeater input for a repeater implementing the gain control methodaccording to one embodiment of the present invention.

FIG. 5A is a flow chart illustrating the gain control method for awireless repeater according to one embodiment of the present invention.

FIG. 5B is a block diagram of a repeater according to one embodiment ofthe present invention.

FIG. 5C is a block diagram of a repeater according to another embodimentof the present invention.

FIG. 6 is a block diagram of a repeater incorporating a gain controlunit according to one embodiment of the present invention

FIG. 7 illustrates the use of a single slow gain control metric to coverthe entire loop gain region of interest.

FIG. 8 illustrates the use of a slow and a fast gain control metric tomonitor the entire loop gain region of interest according to oneembodiment of the present invention.

FIG. 9 is a block diagram of a gain metric calculator in a gain controlunit of a repeater according to one embodiment of the present invention.

FIG. 10 illustrates the computation of the correlation value R_(i) ofthe fast and slow metric using shorter and longer coherent integratetime according to one embodiment of the present invention.

FIG. 11 illustrates the computation of the normalization value S_(i) ofthe fast and slow metric using shorter and longer coherent integratetime according to one embodiment of the present invention.

FIG. 12 illustrates the computation of the fast and slow metric usingthe correlation and normalization values in FIG. 10 and FIG. 11according to one embodiment of the present invention.

FIG. 13 is a block diagram of a repeater employing echo cancellationillustrating the gain control method according to an alternateembodiment of the present invention.

FIG. 14 illustrates the mathematical mode of a repeater incorporatingthe gain control method according to an alternate embodiment of thepresent invention.

FIG. 15 is a block diagram of a repeater without echo cancellation andimplementing the gain control method according to one embodiment of thepresent invention.

FIG. 16 illustrates the update operation for the correlation term R atlag r according to one embodiment of the present invention.

FIG. 17 illustrates the update operation for the normalization term S atlag r according to one embodiment of the present invention.

FIG. 18 is a diagram illustrating the gain adjustment control zonesaccording to one embodiment of the present invention.

FIG. 19 is a flowchart illustrating the gain adjustment stepping controlmethod as applied to the repeater of FIG. 6 implementing multiple metricgain control according to one embodiment of the present invention.

FIG. 20 illustrates the computation of the metric variance over delaylags and over time according to one embodiment of the present invention.

FIG. 21 is a flowchart illustrating the gain control metric pruningmethod implemented in a gain control algorithm according to oneembodiment of the present invention.

FIG. 22 is a flowchart illustrating the gain control metric pruningmethod implemented in a gain control algorithm according to an alternateembodiment of the present invention.

DETAILED DESCRIPTION

The nature, objectives, and advantages of the disclosed method andapparatus will become more apparent to those skilled in the art afterconsidering the following detailed description in connection with theaccompanying drawings.

Prior art repeaters such as those described above may providesignificant advantages for cellular telephone or similar networks.However, existing repeater configurations may not be suitable for someapplications. For example, existing repeater configurations may not besuitable for indoor coverage applications (e.g., repeating signals for aresidence or business environment) which may be more difficult to obtainthe desired isolation between the repeater's antennas. Moreover, in sometraditional repeater implementations, the target is to achieve as high again as reasonable while maintaining a stable feedback loop (loop gainless than unity). However, increasing the repeater gain rendersisolation more difficult due to the increased signal leaking back intothe donor antenna. In general, loop stability demands require that thesignal leaking back into the donor antenna from the coverage antenna bemuch lower than the remote signal (the signal to be repeated). Themaximum achievable signal to interference/noise ratio (SINR) at theoutput of the repeater is then the same as the SINR of the remote signalat the input to the repeater. High gain and improved isolation form twodemands required for modern day repeaters, especially those for indoorapplications.

Systems and techniques herein provide for wireless repeaters withimproved isolation between the repeaters' donor antenna (“the receivingantenna” for the example of a forward link transmission) and thecoverage antenna (“the transmitting antenna” for forward linktransmissions). Furthermore, in some embodiments, systems and techniquesherein provide for a unique repeater design employing interferencecancellation or echo cancellation to significantly improve theisolation. In some embodiments, the interference cancellation and echocancellation are realized using improved channel estimation techniquesprovided herein for accurate estimation of the channel. Effective echocancellation requires very accurate channel estimation of the leakagechannel. In general, the more accurate the channel estimate, the higherthe cancellation and hence the higher the effective isolation. Herein,“interference cancellation” or “echo cancellation” refers to techniquesthat reduce or eliminate the amount of leakage signal between repeaterantennas; that is, “interference cancellation” refers to cancellation ofan estimated leakage signal, which provides for partial or completecancellation of the actual leakage signal.

According to another aspect of the present invention, systems andtechniques herein provide for a unique wireless repeater designemploying gain control techniques for enhancing the stability of therepeater system. In some embodiments, a metric for measuring thestability of the repeater system is provided. The gain of the repeateris controlled based on the value of the metric as an indicator ofstability. For example, in the event of large signal dynamics, a metric,such as the loop gain, becomes degraded and the gain will be reduced tokeep the repeater system stable. The gain control methods and systemscan be advantageously applied to repeaters employing interferencecancellation or repeaters not employing interference cancellation.

Lastly, according to yet another aspect of the present invention,systems and techniques herein provide for improving wireless repeaterperformance in a multi-repeater environment. In some embodiments,systems and techniques that facilitate inter-repeater communication areprovided. In other embodiments, systems and techniques for suppressinginterference and reducing delay spread from neighboring repeaters areprovided.

FIG. 2 shows a diagram of an operating environment 200 for a repeater210 according to embodiments of the current disclosure. The example ofFIG. 2 illustrates forward link transmissions; i.e., a remote signal 140from a base station 225 is intended for a mobile device 230. A repeater,such as repeater 210, may be used in environment 200 if an un-repeatedsignal along the path 227 between base station 225 and mobile device 230would not provide sufficient signal for effective voice and/or datacommunications received at mobile device 230. Repeater 210 with a gain Gand a delay A is configured to repeat a signal received from basestation 225 on a donor antenna 215 to mobile device 230 using a serverantenna 220. Repeater 210 includes forward link circuitry for amplifyingand transmitting signals received from the base station 225 to mobiledevice 230 through donor antenna 215 and server antenna 220. Repeater210 may also include reverse link circuitry for amplifying andtransmitting signals from mobile device 230 back to base station 225. Atrepeater 210, the remote signal s(t) is received as an input signal andthe remote signal s(t) is repeated as a repeated or amplified signaly(t) where y(t)=√{square root over (G)}s(t−Δ). Ideally, the gain G wouldbe large, the inherent delay Δ of the repeater would be small, the inputSINR would be maintained at the output of repeater 210 (this can be ofparticular importance for data traffic support), and only desiredcarriers would be amplified.

In practice, the gain of repeater 210 is limited by the isolationbetween donor antenna 215 and server antenna 220. If the gain is toolarge, the repeater can become unstable due to signal leakage. Signalleakage refers to the phenomenon where a portion of the signal that istransmitted from one antenna (in FIG. 2, server antenna 220) is receivedby the other antenna (in FIG. 2, donor antenna 215), as shown by thefeedback path 222 in FIG. 2. In other words, signal leakage is a resultof the transmitted signal not being totally blocked by antenna isolationbetween the receiving and transmitting antennas. Without interferencecancellation or other techniques, the repeater would amplify thisfeedback signal, also referred to as the leakage signal, as part of itsnormal operation, and the amplified feedback signal would again betransmitted by server antenna 220. The repeated transmission of theamplified feedback signal due to signal leakage and high repeater gaincan lead to repeater instability. Additionally, signal processing inrepeater 210 has an inherent non-negligible delay A. The output SINR ofthe repeater is dependent on RF non-linearities and other signalprocessing. Thus, the aforementioned ideal repeater operationalcharacteristics are often not attained. Finally, in practice, thedesired carriers can vary depending on the operating environment ormarket in which the repeater is deployed. It is not always possible toprovide a repeater that amplifies only the desired carriers.

In embodiments of the current disclosure, a repeater suitable for indoorcoverage (e.g., business, residential, or similar use) is provided. Therepeater has an active gain of about 70 dB or greater which is anexample of a sufficient gain for coverage in a moderately sizedresidence. Furthermore, the repeater has a loop gain of less than onefor stability (loop gain being referred to as the gain of the feedbackloop between the transmitting antenna and the receiving antenna) and asufficient amount of margin for stability and low output noise floor. Insome embodiments, the repeater has a total isolation of greater than 80dB. In some embodiments, the repeater employs interference/echocancellation to achieve a high level of active isolation, which issignificantly more challenging than the requirements of availablerepeaters.

Some techniques of the current disclosure utilize channel estimation toenable the required level of echo cancellation. By estimating thefeedback channel (the channel between the antennas) to a sufficientdegree of accuracy, the residual error, post echo cancellation, can besufficiently below the remote signal to realize the desired loop gainmargin for stability.

The communication system in which the repeater of the present inventioncan be deployed includes various wireless communication networks basedon infrared, radio, and/or microwave technology. Such networks caninclude, for example, a wireless wide area network (WWAN), a wirelesslocal area network (WLAN), a wireless personal area network (WPAN), andso on. A WWAN may be a Code Division Multiple Access (CDMA) network, aTime Division Multiple Access (TDMA) network, a Frequency DivisionMultiple Access (FDMA) network, an Orthogonal Frequency DivisionMultiple Access (OFDMA) network, a Single-Carrier Frequency DivisionMultiple Access (SC-FDMA) network, and so on. A CDMA network mayimplement one or more radio access technologies (RATs) such as CDMA2000,Wideband-CDMA (W-CDMA), and so on. CDMA2000 includes IS-95, IS-2000, andIS-856 standards. A TDMA network may implement Global System for MobileCommunications (GSM), Digital Advanced Mobile Phone System (D-AMPS), orsome other RAT. GSM and W-CDMA are described in documents from aconsortium named “3rd Generation Partnership Project” (3GPP). CDMA2000is described in documents from a consortium named “3rd GenerationPartnership Project 2” (3GPP2). 3GPP and 3GPP2 documents are publiclyavailable. A WLAN may be an IEEE 802.11x network, and a WPAN may be aBluetooth network, an IEEE 802.15x, or some other type of network. Thesystems and techniques described herein may also be used for anycombination of WWAN, WLAN and/or WPAN.

Gain Control Techniques

According to embodiments of the present invention, gain controltechniques for establishing the optimal gain value for a repeater aredescribed. The gain control techniques described herein apply torepeaters implementing echo cancellation or repeaters not implementingecho cancellation. To keep repeater operation stable, control of theloop gain of the repeater is critical. The loop gain of the repeater maychange suddenly because of sudden changes in the signal levels in thechannel. Methods to stabilize the repeater system in the presence oflarge signal dynamics are desired.

In one embodiment, a gain control metric is established as an indicatorof the stability of the repeater. The gain control metric is monitoredand when the metric degrades, the gain of the repeater is decreased tomaintain the stability of the repeater. In operation, the gain controlmetric is monitored continuously and the gain of the repeater isadjusted up and down to output as high a gain as possible while at thesame time to maintain system stability and required output SINR.

1. Repeater Gain Control Optimizing SINR and Data Rate

In traditional repeater implementations, the target is to achieve ashigh a gain as possible while ensuring that the feedback loop remainsstable (loop gain less than unity). High gain level provides maximumpossible coverage area. Loop stability demands require that the signalleaking back into the donor antenna from the coverage antenna istypically much lower than the remote signal (the signal to be repeated).The maximum achievable SINR at the output of the repeater is then thesame as the SINR of the remote signal at the input to the repeater.

The repeater gain may be increased significantly through the use ofinterference cancellation techniques. With these techniques, the signalleaking back from the coverage antenna into the donor antenna is treatedas interference and cancelled through baseband techniques, therebyallowing higher repeater gain to be used. Higher repeater gain is verydesirable since it increases the coverage area of the repeater. In somecases, when the gain is high, the signal leaking into the donor antennafrom the coverage antenna can be significantly larger than the remotesignal. However, RF distortions, such as due to quantization, at thereceiver are dependent on the received signal. In the case where thefeedback signal is significantly larger than the remote signal, thelarge feedback signal introduces a noise floor to the remote signal. Inother words, a large feedback signal introduces a floor on theachievable SINR at the coverage antenna output even if the interferencecancellation works ideally, i.e. even if the entire feedback signal iscancelled. Thus, even if the SINR of the remote signal (depending on RFconditions where the repeater is deployed) is very large, the SINR atthe output of the repeater is limited by the error floor introduced atthe repeater input.

FIG. 3 illustrates the effect of feedback signal on the noise floor atthe repeater input. In the present illustration, the repeater gain is 80dB. Assuming that the remote signal is at −70 dB, the output signal istherefore at 10 dB. Further assuming that the antenna isolation betweenthe transmitting and receiving antennas is 40 dB, the feedback signalfrom the transmitting antenna to the receiving antenna will therefore beat −30 dB. When the RF distortions/quantization floor is 50 dB lowerthan the feedback signal, the noise floor will therefore be at −80 dBwhich introduces a SINR floor of 10 dB only. That is, the noise floor isonly 10 dB lower than the remote signal. The low SINR is a problem whenthe repeater supports wireless data usage. The large repeater gainimplies that the coverage area will be large but the cap on theachievable SINR implies a cap on the maximum achievable data ratethrough this repeater. Depending on the usage requirements (e.g., highdata rates) of the device communicating through this repeater, thelimitation on the data rate may not be acceptable.

According to one aspect of the present invention, a method to controlthe repeater gain in a wireless repeater adjusts the repeater gain basedon the output signal-to-interference/noise ratio (SINR) and the datarate requirements. In one embodiment, the input SINR is used anindicator of the output SINR and as a measure of the noise floor and therepeater gain is adjusted up (increased) or down (decreased) as afunction of the desired noise tolerance and the desired data rate. Inother words, the gain and the SINR of the repeater are traded off tooptimize the desired coverage area versus data rate. For instance, whenmore noise or a lower SINR can be tolerated, a higher repeater gainsetting is used to realize a larger coverage area at the expense of alower data rate due to the lower SINR. On the other hand, when lessnoise or a higher SINR is desired, a lower repeater gain setting is usedwhich reduces the coverage area but increases the data rate.

FIG. 4 illustrates the effect of feedback signal on the noise floor atthe repeater input for a repeater implementing the gain control methodaccording to one embodiment of the present invention. In the embodimentshown in FIG. 4, the gain of the repeater is reduced to 70 dB. While thecoverage area of the repeater is reduced due to the reduced gain, theachievable SINR increased to 20 dB, thereby allowing a higher data rateto be used.

More specifically, reducing the overall gain of the repeater reduces thefeedback signal power. This in turn reduces the noise floors caused bythe RF distortions and quantization effects and thereby allows higherachievable SINR. A higher data rate can be used due to the higher SINR.The achievable SINR is only limited by the SINR of the remote signal. Inthe embodiment shown in FIG. 4, an increase of 10 dB of SINR is realizedwith a 10 dB reduction in gain as compared to the embodiment shown inFIG. 3. However, reducing the repeater gain also reduces the coveragearea. Depending on the repeater usage, a reduction in the coverage areamay be acceptable to the end user. The gain control method of thepresent invention allows the end user to trade-off repeater coveragearea with achievable output SINR (which corresponds to the maximum datarate). The gain control method of the present invention is especiallyuseful for wireless data traffic where the end user may be willing tosacrifice coverage area when using applications that demand higher datarates.

The gain control method of the present invention will now be describedin detail with reference to FIGS. 5A and 5B. FIG. 5A is a flow chartillustrating the gain control method for a wireless repeater accordingto one embodiment of the present invention. FIG. 5B is a block diagramof a repeater according to one embodiment of the present invention.Referring to FIGS. 5A and 5B, a repeater 350 receives an input signal ona donor antenna 315 (step 302). Repeater 350 also transmit an outputsignal on a server antenna 320 (step 304). Repeater 350 includes an echocanceller 352 for implementing cancellation of the feedback signal.Repeater 350 amplifies the input signal at a gain stage 356 providing avariable gain G. Variable gain G is set by a gain control block 354. Inoperation, the gain control method of the present invention determinesthe input SINR (step 305) and the output SINR of the repeater (step306). In one embodiment, the input SINR is used as an indicator of theoutput SINR and the noise floors added by the repeater. Then, therepeater gain is adjusted up or down to obtain the desired coverage areaand the achievable data rate (step 308). The repeater gain is decreasedto reduce the coverage area but increase the data rate (step 310). Bydecreasing the gain, the achievable SINR is increased. The repeater gainis increased to enlarge the coverage area but decrease the data rate(step 312). By increasing the gain, the achievable SINR is decreased. Inthis manner, the repeater gain and the coverage area of the repeater istraded off to optimize the SINR at the input or output of the repeaterand optimize the data rate used by the repeater.

FIG. 5C is a block diagram of a repeater according to another embodimentof the present invention. FIG. 5C illustrates a functional block diagramof a repeater 360 including a mobile station modem (MSM) providingcommunication on donor and server antennas 315, 320, a processor 364 forperforming variable repeater operations and a memory 366 for storingdata. In the present embodiment, the MSM (mobile station modem) at therepeater is used to give an estimate of the input SINR. In otherembodiments, other methods for estimating the input or output SINR ofthe repeater can be used.

2. Multi-Metric Gain Control

According to embodiments of the present invention, a repeater includes again control block employing multiple metrics as indicators of systemstabilities where the multiple metrics are monitored for use incontrolling the gain of the repeater. The gain control block implementsa gain control method where the repeater gain is controlled based on themultiple metrics. FIG. 6 is a block diagram of a repeater incorporatinga gain control block according to one embodiment of the presentinvention. In the present embodiment, the gain control block isimplemented in an echo cancelling repeater. In other embodiments, thegain control block can be implemented in a repeater without echocancellation to provide gain control based on multiple metrics asdescribed below.

Referring to FIG. 6, an echo-cancellation repeater 410 receives a remotesignal x[k] to be repeated on a donor antenna (denoted as input node440) and generates an output signal y[k] to be transmitted on a serverantenna (denoted as output node 470). Signal leakage from the serverantenna back to the donor antenna causes part of the output signal y[k]to be leaked back and added to the remote signal before being receivedby the repeater. The signal leakage is represented as a feedback channelh[k], denoted as a signal path 454 between output node 470 and the inputnode 440. Thus, repeater 410 actually receives as the input signal on anode 443 a receive signal q[k] being the sum of the remote signal x[k]and the feedback signal w[k]. The feedback channel h[k] thus form afeedback loop in repeater 410 between the donor antenna and the serverantenna. A summer 442 in FIG. 6 is symbolic only to illustrate thesignal components of receive signals q[k] and does not represent anactual signal summer in the operating environment of repeater 410.

Repeater 410, being an echo-cancellation repeater, operates to estimatethe feedback signal w[k] in order to cancel out the undesired feedbacksignal component in the receive signal (“the input signal”). To thatend, repeater 410 includes an echo canceller formed by a summer 444 anda channel estimation block 450. The receive signal q[k] is coupled tosummer 444 which operates to subtract a feedback signal estimate ŵ[k]from the receive signal q[k]. As long as the feedback signal estimateŵ[k] is accurate, the undesired feedback signal is removed from thereceive signal and echo cancellation is realized. In the presentembodiment, the post cancellation signal p[k] is coupled to a variablegain stage 458 providing a gain of G to the post cancellation signal.Gain stage 458 generates the output signal y[k] on the output node 470for transmission on the server antenna. FIG. 6 illustrates only elementsthat are relevant to operation of the gain control method of the presentinvention. Repeater 410 may include other elements not shown in FIG. 6but known in the art to realize the complete repeater operation.

Feedback signal estimate ŵ[k] is generated based on a feedback channelestimate ĥ[k] where the feedback channel estimate is generated by achannel estimation block 450. In the present embodiment, the channelestimation block 450 takes as an input signal the receive signal q[k]and uses the echo-cancelled signal p[k] as the pilot signal or thereference signal for channel estimation to generate the feedback channelestimate ĥ[k]. Then, echo canceller computes the feedback signalestimate ŵ[k] based on the feedback channel estimate ĥ[k]. Morespecifically, the feedback signal estimate w[k] is obtained byconvolving the feedback channel estimate ĥ[k] with the pilot signal p[k](i.e., ŵ[k]=ĥ[k]

p[k]). The feedback signal estimate ŵ[k] is used for echo cancellationat summer 444. More specifically, the feedback signal estimate ŵ[k] issubtracted from the receive signal q[k] to generate the echo-cancelledsignal p[k]. It is imperative to note that FIG. 6 illustrates one methodfor implementing echo cancellation in a repeater. FIG. 6 is illustrativeand is not intended to be limiting. An echo cancelling wireless repeaterof the present invention can implement other methods for echocancellation. The exact method of echo cancellation used in the repeateris not critical to the practice of the present invention.

Repeater 410 incorporates a gain control block 447 for adjusting thevariable gain value G provided by gain stage 458. Gain control block 447receives a gain control input signal which can be taken from anywhere inthe feedback loop of the repeater. More specifically, the gain controlinput signal can be taken before echo cancellation or after echocancellation in an echo cancelling repeater. In the present embodiment,the gain control input signal is taken as the receive signal q[k] butthis is illustrative only. In practice, the exact location of where thegain control input signal is taken is not critical to the practice ofthe present invention. In other embodiments, the repeater does notimplement echo cancellation and the gain control block 447 receives again control input signal which can be a signal anywhere in the feedbackloop of the repeater. Again, the exact location of where the gaincontrol input signal is taken is not critical to the practice of thepresent invention. Accordingly, in the following description, the term“gain control input signal” refers to the input signal provided to thegain control block of the repeater and can be a signal taken at anypoint in the feedback loop of the repeater, including before echocancellation, after echo cancellation, or any point in the feedback loopin a repeater not implementing echo cancellation.

Gain control block 447 includes a gain metric calculator 460 forreceiving the gain control input signal and calculating and generating aset of gain control metrics. In one embodiment, two gain control metricsare used. Gain control block 447 further includes a gain controlalgorithm block 462 receiving the gain control metrics from the gainmetric calculator 460. The gain control algorithm block 462 providescontrol of the variable gain G of the gain stage 458 in repeater 410.

The derivation of a first gain control metric is described withreference to FIG. 6. First, a complex signal segment of length N at timei from the repeater feedback control loop is intercepted or received andused as the gain control input signal:

${{r_{i}\lbrack n\rbrack} = \frac{q\left\lbrack {n + i} \right\rbrack}{\sqrt{\sum\limits_{i = 0}^{N - 1}{{q\left\lbrack {n + i} \right\rbrack}}^{2}}}},{0 \leq n < {N.}}$

In the present embodiment, the complex signal r_(i)[n] is taken at node443 which is before echo cancellation. In other embodiments, the complexsignal r_(i)[n] can be taken at other locations in the feedback controlloop, such as after echo cancellation. In the present description, therepeater feedback control loop (also referred to as the “control loop”)refers to the feedback loop inherently formed in the repeater betweenthe transmitting antenna and the receiving antenna as a result of thefeedback channel from the transmitting antenna to the receiving antenna.The gain of the feedback loop (“the loop gain”) is measured andcontrolled to maintain loop stability.

The gain metric calculator 460 monitors the growth of this signalcomponent in the loop by trying to pick up replicas of the signal, as aresult of the leakage from the transmit antenna to the receive antenna.Searching in a search window W after time N_(delay) at τεW≡{0, 1, L,N_(tap)−1} gives

${g_{i}(\tau)} = {\frac{{{\sum\limits_{n = 0}^{N - 1}{{r^{*}\lbrack n\rbrack}{q\left\lbrack {n + i + N_{delay} + \tau} \right\rbrack}}}}^{2}}{\sum\limits_{n = 0}^{N - 1}{{q\left\lbrack {n + i} \right\rbrack}}^{2}} = {\left( \frac{{\sum\limits_{n = 0}^{N - 1}{{q^{*}\left\lbrack {n + i} \right\rbrack}{q\left\lbrack {n + i + N_{delay} + \tau} \right\rbrack}}}}{\sum\limits_{n = 0}^{N - 1}{{q\left\lbrack {n + i} \right\rbrack}}^{2}} \right)^{2}.}}$

The loop gain metric g_(i)(τ) given above is essentially the loop gainwhich is an indicator of system stability. The loop gain metric g_(i)(τ)given above computes the loop gain for each channel tap τ and isreferred hereinafter as the “tap-specific gain control metric.” Thetap-specific gain control metric g_(i)(τ), when summed over all channeltaps, can be used for adjusting the gain G_(i) of the variable gainstage 458 in a way such that:

$g_{i} = {{\sum\limits_{\tau \in {\{{0,1,L,{N_{tap} - 1}}\}}}{{\alpha (\tau)}{g_{i}(\tau)}}} \approx \delta < 1.}$

That is, the tap-specific gain control metric g_(i)(τ) is measured andsummed over all of the desired channel taps and the summed value is thegain control metric g_(i) for the repeater which is maintained to beabout the value δ which is less than 1. Typically, δ is determined bythe output SINR requirement. In one embodiment, δ is selected to be −10dB to −20 dB in accordance with the required output SINR. Also, fortypical repeater operation, the total loop gain has to be less than 1 (0dB) for stability. In one embodiment, δ is selected to be −20 dB whenthe required output SINR is in the range of 20 dB. In the aboveequation, the gain control metric g_(i) is computed as a linearcombination of the tap-specific gain control metric g_(i)(τ) over allchannel taps, each tap-specific gain control metric g_(i)(τ) beingmultiplied by its own coefficient α(τ), which can be 1 for a straightsummation of the tap-specific gain control metric terms or other valuesfor other forms of linear combination. In other embodiments, the gaincontrol metric g_(i) can be computed as a non-linear combination of thetap-specific gain control metric g_(i)(τ) over all of the desiredchannel taps.

In one embodiment, the gain control metric g_(i) shown above issimulated in the absence of echo cancellation and gain control and withwhite input signal. A sampling rate of 30 MHz is used and a delay of thesearch window of N_(delay)=30×5 is used. The length of the search windowis N_(tap)=64. The integration length is N=30×10 samples. The gaincontrol metric is able to accurately estimate the actual loop gain forloop gains higher than −20 dB. However, the estimation noise baselineprevents the gain control metric from estimating loop gain levels lowerthan −20 dB. In the case where the target loop gain level is −20 dB, ametric with lower noise baseline is desired. A lower noise baseline canbe obtained by increasing the integration length N. In one embodiment,the integration length N is increased to 30×200 samples. When the gaincontrol metric uses an integration length of N=30×200 samples, theincreased integration length allows the gain control metric to estimateloop gain accurately above −30 dB.

In one embodiment of the gain control method of the present invention, asingle gain control metric having a long integration length is used. Thelong integration length increases the response time and therefore a gaincontrol metric with a long integration length is referred herein as a“slow metric”. For instance, in one embodiment, a gain control metricwith integration length of N=30×200 samples (the “slow metric”) is usedfor gain control with loop gain target of −20 dB. FIG. 7 illustrates theuse of a single slow gain control metric to monitor the entire loop gainregion of interest. However, the slow metric uses a long integrationlength and therefore has a long response time. In one example, anintegration length of N=30×200 samples requires a response time of 200μsec. Thus, while the slow metric is advantageous for slow adjustment,the slow metric may be too slow to respond to abrupt disturbances thatmay throw the repeater out of the stable region.

According to other embodiments of the gain control method of the presentinvention, the gain control method uses multiple gain control metrics tomonitor loop stability for controlling the repeater gain. In oneembodiment, the gain control method uses a “fast metric” in conjunctionwith a “slow metric” to monitor the entire loop gain region of interest.As described above, a “slow metric” refers to a gain control metrichaving a long integration length and therefore a slow response time.However, the slow metric uses a large number of samples and hence isslow but very accurate. On the other hand, a “fast metric” refers to again control metric having a short integration length and therefore afast response time. However, the fast metric uses a small number ofsamples and hence the fast metric is fast but less accurate. In oneembodiment, the fast metric with a fast response time is used to monitorthe critical region of loop gain near 0 dB where the feedback controlloop of the repeater approaches instability. In this critical region, afast response is desired to allow the repeater gain to be adjustedquickly to ensure loop stability. On the other hand, the slow metricwith a slow response time is used to monitor the normal stable region ofloop gain where more accurate loop gain measurements are desired. In thepresent description, the integration length is defined as the sum of thecoherent integration time and the non-coherent integration time.

FIG. 8 illustrates the use of a slow and a fast gain control metric tomonitor the entire loop gain region of interest according to oneembodiment of the present invention. Referring to FIG. 8, a fast metricwith integration length N=30×10 samples and a response time of 10 μsecis used to monitor the loop gain in the critical region of loop gain ator higher than −10 dB. The region of loop gain at or higher than −10 dBrepresents loop gain that approaches 0 dB (or loop gain equal to orgreater than 1) where loop instability can result. The fast metricensures fast gain control response for the repeater. In conjunction withthe fast metric, a slow metric with integration length N=30×200 samplesand a response time of 200 μsec is used to monitor the loop gain in thenormal stable region of loop gain greater than −10 dB and around thetarget loop gain of −20 dB. When the loop gain is sufficiently away fromthe instability region, accurate loop gain measurements are desired toallow fine adjustment of the repeater gain to the desired gain value.The slow metric provides more accurate loop gain measurements to allowthe repeater gain to be accurately controlled. By using two metrics tomonitor and measure the entire loop gain region of interest, accurategain control from −30 dB to −10 dB is realized and fast gain controlabove −10 dB is realized.

FIG. 9 is a block diagram of a gain metric calculator in a gain controlblock of a repeater according to one embodiment of the presentinvention. Referring to FIG. 9, a gain metric calculator 560 takes asinput a gain control input signal and generates as outputs a fast metricand a slow metric. The fast metric and the slow metric are used by thegain control algorithm block in the gain control block to generate thecontrol signal for controlling the variable gain of the repeater, asshown in FIG. 6. The gain metric calculator 560 is constructed asfollows. The receive signal q[n] is used as the gain control inputsignal for gain metric calculation. At block 570, the receive signalq[n] is used to compute a normalized correlation (complex) valueη_(i)(τ) at a channel tap τ which detects the feedback signal at channeltap τ. The normalized correlation (complex) value η_(i)(τ) is given as:

${\eta_{i}(\tau)} = {\frac{{\sum\limits_{n = 0}^{N - 1}{{q^{*}\lbrack n\rbrack}{q\left\lbrack {n + N_{delay} + \tau} \right\rbrack}}}}{\sum\limits_{n = 0}^{N - 1}{{q\lbrack n\rbrack}}^{2}}.}$

Then at block 572, the normalized correlation value η_(i)(τ) is squaredand summed over all channel taps to generate an estimated gain controlmetric value g_(i). More specifically, squaring of the complexnormalized correlation value η_(i)(τ) gives the feedback energy relativeto the output signal energy at channel tap τ while summation of squaredη_(i)(τ) at all corresponding channel taps gives the total relativefeedback energy which is an estimate of the loop gain of the feedbackloop. The estimated gain control metric g_(i) is given as:

$g_{i} = {\sum\limits_{\tau}{{{\eta_{i}(\tau)}}^{2}.}}$

After computing the gain control metric g_(i), two infinite impulseresponse (IIR) filters 574, 576 are used in parallel to generate thefast metric and the slow metric in parallel. In other embodiments, othertypes of filters, such as FIR, can also be used. IIR filter 574 is usedto generate the fast metric and uses a delay value of D_(fast) while IIRfilter 576 is used to generate the slow metric and uses a delay value ofD_(slow), where D_(slow) is much greater than D_(fast). The delay valuesare determined by the filter bandwidth. The output of IIR filter 574 issampled by a switch 575 having a sampling time T_(sample) equals toD_(fast). The output of IIR filter 576 is sampled by a switch 577 havinga sampling time T_(sample) equals to D_(slow). The non-coherentintegration time is determined by the delay values D_(fast) or D_(slow).

In FIG. 9, the receive signal q[n] before echo cancellation is used asthe gain control input signal for gain metric calculation. In otherembodiments, other signal from the feedback loop, such as after echocancellation, can be used as the gain control input signal for gainmetric calculation.

In the present embodiment, gain metric calculator 560 includes a biasestimator 590 for removing any undesired bias in the metric calculation.The detail operation of bias estimator 590 will be described in moredetail below. In general, bias estimator 590 takes as input thenormalized correlation value (complex) η_(i)(τ) and generates a biasvalue which is to be subtracted from the fast and slow metric outputsfrom the IIR filters. More specifically, the sampled output of IIRfilter 574 is coupled to a summer 580 and the sampled output of IIRfilter 576 is coupled to a summer 582. At summers 580 and 582, theestimated bias value is subtracted from sampled outputs to generate thefast metric and slow metric respectively.

As thus constructed, gain metric calculator 560 provides two gaincontrol metrics for use by the gain control algorithm. The gain controlalgorithm selects the desired metrics based on the loop gain value. Ifthe loop gain is around or close to the instability region (such asaround −5 dB), then the fast metric is used to obtain fast gain controlresponse. If the loop gain is in the normal stable region (such asaround −20 dB), then the slow metric is used to obtain more accurategain control.

In the above description, the slow and fast metrics are realized using alarge number and a small number of samples, respectively. According toanother embodiment of the gain control method of the present invention,the fast and slow metrics are realized using different coherentintegration times without using noncoherent filtering. For the fastmetric, a shorter coherent integration time is used. For the slowmetric, a longer coherent integration time is used. FIG. 10 illustratesthe computation of a correlation value R_(i) of the fast and slowmetrics using shorter and longer coherent integration times according toone embodiment of the present invention. More specifically, correlationvalue R₁[τ] corresponds to the slow metric where a longer coherentintegration time is used while correlation value R₂[τ] corresponds tothe fast metric where a shorter coherent integration time is used. FIG.11 illustrates the computation of the normalization value S_(i) of thefast and slow metrics using shorter and longer coherent integrate timesaccording to one embodiment of the present invention. More specifically,normalization value S₁ corresponds to the slow metric where a longercoherent integration time is used while normalization value S₂corresponds to the fast metric where a shorter coherent integration timeis used. FIG. 12 illustrates the computation of the fast metric g² andthe slow metric g¹ using the correlation values R_(i) and thenormalization values S in FIG. 10 and FIG. 11 according to oneembodiment of the present invention.

3. Repeater Stability Indicator

A repeater should be maintained unconditionally stable. A metric thatindicates the stability of a repeater is therefore desirable. Accordingto embodiments of the present invention, a repeater stability monitoringmethod and apparatus operate by intercepting certain period of a signalfrom the repeater feedback loop and monitor the “growth” of that signalcomponent in the loop for a period of time. If the signal component inthe feedback loop dies down, the repeater system is stable. As describedabove, the repeater feedback loop (or “control loop”) refers to thefeedback loop inherently formed in the repeater between the transmittingantenna and the receiving antenna. The gain of the feedback loop (“theloop gain”) is measured and controlled to maintain loop stability.

FIG. 13 is a block diagram of a repeater employing echo cancellationillustrating the repeater stability monitoring and gain control methodaccording to one embodiment of the present invention. FIG. 14illustrates the mathematical mode of the repeater incorporating therepeater stability monitoring and gain control method according to oneembodiment of the present invention. Referring to FIG. 13, anecho-cancellation repeater 610 receives a remote signal x[k] to berepeated on a donor antenna (denoted as input node 640) and generates anoutput signal y[k] to be transmitted on a server antenna (denoted asoutput node 670). Signal leakage from the server antenna back to thedonor antenna causes part of the output signal y[k] to be leaked backand added to the remote signal before being received by the repeater.The signal leakage is represented as a feedback channel h[k], denoted asa signal path 654 between output node 670 and the input node 640. Thus,repeater 610 actually receives as the input signal on a node 643 areceive signal q[k] being the sum of the remote signal x[k] and thefeedback signal w[k]. The feedback channel h[k] thus form a feedbackloop in repeater 610 between the donor antenna and the server antenna. Asummer 642 in FIG. 13 is symbolic only to illustrate the signalcomponents of receive signals q[k] and does not represent an actualsignal summer in the operating environment of repeater 610.

Repeater 610, being an echo-cancellation repeater, operates to estimatethe feedback signal w[k] in order to cancel out the undesired feedbacksignal component in the receive signal (“the input signal”). To thatend, repeater 610 includes an echo canceller 680 which includes a summer644 and a channel estimation block 650 (FIG. 14). The receive signalq[k] is coupled to summer 644 which operates to subtract a feedbacksignal estimate ŵ[k] from the receive signal q[k]. As long as thefeedback signal estimate ŵ[k] is accurate, the undesired feedback signalis removed from the receive signal and echo cancellation is realized. Inthe present embodiment, the post cancellation signal p[k] (node 645) iscoupled to a variable gain stage 658 providing a gain of G to the postcancellation signal. Gain stage 658 generates the output signal y[k] onthe output node 670 for transmission on the server antenna. FIG. 13illustrates only elements that are relevant to operation of the gaincontrol method of the present invention. Repeater 610 may include otherelements not shown in FIG. 13 but known in the art to realize thecomplete repeater operation.

Feedback signal estimate ŵ[k] is generated based on a feedback channelestimate ĥ[k] where the feedback channel estimate is generated by achannel estimation block 650. In the present embodiment, the channelestimation block 650 takes as an input signal the receive signal q[k]and uses the echo-cancelled signal p[k] as the pilot signal or thereference signal for channel estimation to generate the feedback channelestimate ĥ[k]. Then, echo canceller computes the feedback signalestimate ŵ[k] based on the feedback channel estimate ĥ[k]. Morespecifically, the feedback signal estimate ŵ[k] is obtained byconvolving the feedback channel estimate ĥ[k] with the pilot signal p[k](i.e., ŵ[k]=ĥ[k]

p[k]). The feedback signal estimate ŵ[k] is used for echo cancellationat summer 644. More specifically, the feedback signal estimate ŵ′ [k] issubtracted from the receive signal q[k] to generate the echo-cancelledsignal p[k]. FIG. 13 illustrates one method for implementing echocancellation. FIG. 13 is intended to be illustrative only and is notintended to be limiting. In other embodiments, other methods forimplementing echo cancellation can be used.

Repeater 610 incorporates a gain control block 647 for adjusting thevariable gain value G provided by gain stage 658. Gain control block 647includes a gain metric calculator 660 for calculating and monitoring again control metric. Gain control block 647 further includes a gaincontrol algorithm block 662 receiving the gain control metrics from thegain metric calculator 660. The gain control algorithm block 662provides control of the variable gain G of the gain stage 658 inrepeater 610. Gain control block 647 receives a gain control inputsignal which can be taken from anywhere in the feedback loop of therepeater. More specifically, the gain control input signal can be takenbefore echo cancellation or after echo cancellation in an echocancelling repeater. In the present embodiment, the gain control inputsignal is taken as the post-cancellation signal p[k] but this isillustrative only. In practice, the exact location of where the gaincontrol input signal is taken is not critical to the practice of thepresent invention. In other embodiments, the repeater does not implementecho cancellation and the gain control block 647 receives a gain controlinput signal which can be a signal anywhere in the feedback loop of therepeater. Again, the exact location of where the gain control inputsignal is taken is not critical to the practice of the presentinvention.

The derivation of the gain control metric is now described withreference to FIG. 13 and FIG. 14. First, in the present embodiment, acomplex signal segment of length N at i from the repeater feedbackcontrol loop (at node 645) is intercepted or received and used as thegain control input signal:

${{r_{i}\lbrack n\rbrack} = \frac{q\left\lbrack {n + i} \right\rbrack}{\sqrt{\sum\limits_{i = 0}^{N - 1}{{q\left\lbrack {n + i} \right\rbrack}}^{2}}}},{0 \leq n < {N.}}$

In other embodiments, the complex signal segment can be taken at otherlocations, such as at node 643 before the echo canceller 680.

The gain metric calculator 660 monitors the growth of this signalcomponent in the loop by trying to pick up replicas of the signal, as aresult of the leakage from the transmit antenna to the receive antenna.Searching in a search window W after time N_(delay) at τεW≡{0, 1, L,N_(tap)−1} gives:

${g_{i}(\tau)} = {\frac{{{\sum\limits_{n = 0}^{N - 1}{{r^{*}\lbrack n\rbrack}{p\left\lbrack {n + i + N_{delay} + \tau} \right\rbrack}}}}^{2}}{\sum\limits_{n = 0}^{N - 1}{{p\left\lbrack {n + i} \right\rbrack}}^{2}} = {\left( \frac{{\sum\limits_{n = 0}^{N - 1}{{p^{*}\left\lbrack {n + i} \right\rbrack}{p\left\lbrack {n + i + N_{delay} + \tau} \right\rbrack}}}}{\sum\limits_{n = 0}^{N - 1}{{p\left\lbrack {n + i} \right\rbrack}}^{2}} \right)^{2}.}}$

The above metric is essentially the loop gain which is an indicator ofsystem stability. The loop gain metric g_(i)(τ) given above computes theloop gain for each channel tap τ and is referred hereinafter as the“tap-specific gain control metric.” The tap-specific gain control metricg_(i)(τ), when summed over all channel taps, can be used for adjustingthe gain G_(i) of the variable gain stage 658 in a way such that:

$g_{i} = {{\sum\limits_{\tau \in {\{{0,1,L,{N_{tap} - 1}}\}}}{{\alpha (\tau)}{g_{i}(\tau)}}} \approx \delta < 1.}$

That is, the tap-specific gain control metric g_(i)(τ) is measured andsummed over a certain time period and the summed value is the gaincontrol metric g_(i) which is maintained to be about the value δ whichis less than 1. Typically, δ is determined by the output SINRrequirement. In one embodiment, δ is selected to be −10 dB to −20 dB inaccordance with the required output SINR. In one embodiment, the gaincontrol metric g_(i) is computed as a linear combination of thetap-specific gain control metric g_(i)(τ) over all channel taps, eachtap-specific gain control metric g_(i)(τ) being multiplied by its owncoefficient α(τ), which can be 1 for a straight summation of thetap-specific gain control metric terms or other values for other formsof linear combination. In other embodiments, the gain control metricg_(i) can be computed as a non-linear combination of the tap-specificgain control metric g_(i)(τ) over all of the desired channel taps.

In one embodiment, the gain control metric g, shown above is simulatedwith a white input signal. A sampling rate of 20 MHz is used and a delayof the search window of N_(delay)=100 is used. The length of the searchwindow is N_(tap)=64. The integration length is N=6400 samples. The gaincontrol metric g, accurately indicates the loop gain levels. The noisebaseline is less than −30 dB for 6400 samples of the correlation period.

In operation, the loop gain of the repeater is estimated by measuringthe residual feedback signal that was not cancelled out by the echocanceller 680. In the case of a non-echo-cancelling repeater, the amountof feedback signal that is left from the antenna isolation is measuredas an estimation of loop gain. The more feedback signal that iscancelled out, the more stable the system is.

In other words, the gain control metric is a measure of the correlationbetween the transmitted signal and the received signal. A largecorrelation indicates a large amount of leakage and less stability. Thegain control algorithm will respond to the gain control metric and lowerthe gain. A small correlation indicates a small feedback signal andincreased stability. The gain control algorithm will respond to the gaincontrol metric and increase the gain. In this manner, the stability ofthe repeater is ensured.

4. Gain Control Metric Computation

The gain control metric g_(i) described above with reference to FIGS. 13and 14 is capable of accurately monitoring the stability of a feedbacksystem such as a repeater. The repeater stability monitoring method andapparatus can be adapted for use in a repeater with echo cancellation orwithout echo cancellation. FIG. 15 is a block diagram of a repeaterwithout echo cancellation and implementing the gain control methodaccording to one embodiment of the present invention. Referring to FIG.15, a repeater 710 receives a remote signal x[k] to be repeated on adonor antenna (denoted as input node 740) and generates an output signaly[k] to be transmitted on a server antenna (denoted as output node 770).Signal leakage from the server antenna back to the donor antenna causespart of the output signal y[k] to be leaked back and added to the remotesignal before being received by the repeater. The signal leakage isrepresented as a feedback channel h[k], denoted as a signal path 754between output node 770 and the input node 740. Thus, repeater 710actually receives as the input signal on a node 743 a receive signalp[k] being the sum of the remote signal x[k] and the feedback signalw[k]. The feedback channel h[k] thus form a feedback loop in repeater710 between the donor antenna and the server antenna. A summer 742 inFIG. 15 is symbolic only to illustrate the signal components of receivesignals p[k] and does not represent an actual signal summer in theoperating environment of repeater 710. The receive signal p[k] iscoupled to a variable gain stage 758 providing a gain of G. Gain stage758 generates the output signal y[k] on the output node 770 fortransmission on the server antenna. FIG. 15 illustrates only elementsthat are relevant to operation of the gain control method of the presentinvention. Repeater 710 may include other elements not shown in FIG. 15but known in the art to realize the complete repeater operation.

Repeater 710 incorporates a gain control block 747 for adjusting thevariable gain value G provided by gain stage 758. Gain control block 747includes a gain metric calculator 760 for calculating and monitoring again control metric. Gain control block 747 further includes a gaincontrol algorithm block 762 receiving the gain control metrics from thegain metric calculator 760. The gain control algorithm block 762provides control of the variable gain G of the gain stage 758 inrepeater 710 based on one or more functions, including at least the gaincontrol metric generated by gain metric calculator 760. Gain controlblock 747 receives a gain control input signal which can be taken fromanywhere in the feedback loop of the repeater. The exact location ofwhere the gain control input signal is taken is not critical to thepractice of the present invention.

As described above, in different embodiments of the present invention, arepeater implements gain control techniques for establishing the optimalgain value for the repeater while maintaining stability. To that end,the repeater includes a gain control block configured to measure andmonitor one or more gain control metrics. The gain control block is alsoconfigured to control the gain of the repeater based at least in part onthe gain control metric(s). In some embodiments, the repeater is an echocancelling repeater (FIG. 13) and the gain control block 647 receives again control input signal which can be taken from anywhere in thefeedback loop of the repeater. More specifically, the gain control inputsignal can be taken before echo cancellation or after echo cancellation.In other embodiments, the repeater does not implement echo cancellation(FIG. 15) and the gain control block 747 receives a gain control inputsignal which can be a signal anywhere in the feedback loop of therepeater. In the following description, the term “gain control inputsignal” refers to the input signal provided to the gain control block ofthe repeater and can be a signal taken at any point in the feedback loopof the repeater, including before echo cancellation, after echocancellation, or without any echo cancellation.

The tap-specific gain control metric g_(i)(τ) at time i and at a channeltap τ described above is repeated here:

${g_{i}(\tau)} = {\left( \frac{{\sum\limits_{n = 0}^{N - 1}{{p^{*}\left\lbrack {n + i} \right\rbrack}{p\left\lbrack {n + i + N_{delay} + \tau} \right\rbrack}}}}{\sum\limits_{n = 0}^{N - 1}{{p\left\lbrack {n + i} \right\rbrack}}^{2}} \right)^{2} = {{\eta_{i}(\tau)}}^{2}}$

The tap-specific gain control metric g_(i)(τ) can be characterized as asquare of a correlation term R_(i) in the numerator divided by anormalization term S_(i) in the denominator, given as:

g _(i)[τ]=(R[τ]/S)² τ=0,1,L,N_(tap)−1,

where τ is the channel taps in time domain associated with the spread ofthe feedback signal in time domain, such as due to multipath effect. Thecorrelation term and the normalization term are each computed for anintegration length of N, that is, for N samples. Furthermore, thecorrelation term represents a correlation of the gain control inputsignal and a delayed version of the gain control input signal while thenormalization term represents the power of the gain control input signalthat is not delayed.

The gain control metric g_(i) is the sum of the tap-specific gaincontrol metric g_(i)(τ) over all the channel taps, given as:

$g_{i} = {{\sum\limits_{\tau \in {ChannelTaps}}{g_{i}(\tau)}} = {\sum\limits_{\tau \in {ChannelTaps}}{{{\eta_{i}(\tau)}}^{2}.}}}$

However, the calculation of the gain control metric is computationallyintensive. For instance, for computing the correlation term R_(i), alarge number of multiplications is required to find the correlationvalue. More specifically, for an integration length of N, each time thecorrelation term needs to be updated, N complex multiplications have tobe carried out; and each time the normalization term needs to beupdated, another N complex multiplications have to be carried out. Forlarge integration length N, the computational complexity can beprohibitive in practice.

According to embodiments of the present invention, a gain control metriccomputation method enables efficient implementation of theabove-described gain control metric. A particular advantage of the gaincontrol metric computation method of the present invention is that thecomplexity of the computation is independent of the integration lengthN, i.e., the complexity of the computation does not increase with theintegration length.

First, in the computation of the correlation term R_(i), at each time i,most of the multiplications are the same as the ones already computed inthe previous time sample except for one. In the present embodiment, aregister is used to hold the correlation value R[τ] at each lag τcomputed for a previous N samples. All the multiplication terms from theprevious time samples are computed and summed and stored in theregister. When a new time sample is introduced, the (N+1)th previoussample becomes the old or obsolete sample. The correlation value R[τ] iscomputed by discarding the multiplication term of the obsolete sampleand adding the multiplication term of the new sample to the storedcorrelation value. As a result, only two multiplications are performedat each time sample—one for the new sample and one for the obsoletesample. The product based on the new sample is added to the stored sumand the product based on the obsolete sample is subtracted from thestored sum to generate the updated correlation value R[τ].

More specifically, the correlation value at each lag τ is given as:

${{R_{i}(\tau)} = {\sum\limits_{n = 0}^{N - 1}{{p^{*}\left\lbrack {n + i} \right\rbrack}{p\left\lbrack {n + i + N_{delay} + \tau} \right\rbrack}}}},{\tau = 0},1,L,{N_{tap} - 1.}$

The correlation value is updated once a new sample is received, asfollows:

$\begin{matrix}{{R_{i + 1}\lbrack\tau\rbrack} = {\sum\limits_{n = 0}^{N - 1}{{p^{*}\left\lbrack {n + i + 1} \right\rbrack}{p\left\lbrack {n + i + 1 + N_{delay} + \tau} \right\rbrack}}}} \\{= {{\sum\limits_{n = 0}^{N - 1}{{p^{*}\left\lbrack {n + i} \right\rbrack}{p\left\lbrack {n + i + N_{delay} + \tau} \right\rbrack}}} +}} \\{{{{p^{*}\left\lbrack {N + i} \right\rbrack}{p\left\lbrack {N + i + N_{delay} + \tau} \right\rbrack}} - {{p^{*}\lbrack i\rbrack}{p\left\lbrack {i + N_{delay} + \tau} \right\rbrack}}}} \\{= {{R_{i}\lbrack\tau\rbrack} + {{p^{*}\left\lbrack {N + i} \right\rbrack}{p\left\lbrack {N + i + N_{delay} + \tau} \right\rbrack}} -}} \\{{{p^{*}\lbrack i\rbrack}{p\left\lbrack {i + N_{delay} + \tau} \right\rbrack}}}\end{matrix}$

As can be observed from the above equation, only two complexmultiplications are required per lag per new sample instead of Nmultiplications required in the conventional computation. FIG. 16illustrates the update operation for the correlation term R[τ] at lag τaccording to one embodiment of the present invention. As shown in FIG.16, a register 780, a complex register, is used to hold the correlationvalue R[τ] at each lag τ computed for a previous N samples. For each newsample, only two complex multiplications are performed per lag per newsample in order to obtain the updated correlation term R[τ]. One complexmultiplication (multiplier 772) is performed to compute the term for theobsolete sample and one complex multiplication (multiplier 774) isperformed to compute the term for the new sample. The product based onthe new sample is added to the stored sum and the product based on theobsolete sample is subtracted from the stored sum (summer 776) togenerate the updated correlation value R[τ].

The normalization term (common to all lags) is updated in a similarfashion as the correlation value. More specifically, the normalizationterm S_(i+1) is updated as follows:

$\begin{matrix}{S_{i + 1} = {\sum\limits_{n = 0}^{N - 1}{{p\left\lbrack {n + i + 1} \right\rbrack}}^{2}}} \\{= {{\sum\limits_{n = 0}^{N - 1}{{p\left\lbrack {n + i} \right\rbrack}}^{2}} + {{p\left\lbrack {N + i} \right\rbrack}}^{2} - {{p\lbrack i\rbrack}}^{2}}} \\{= {S_{i} + {{p\left\lbrack {N + i} \right\rbrack}}^{2} - {{{p\lbrack i\rbrack}}^{2}.}}}\end{matrix}$

FIG. 17 illustrates the update operation for the normalization term Satlag r according to one embodiment of the present invention. As shown inFIG. 17, a register 790 is used to hold the normalization value S[τ] ateach lag τ. All the multiplication terms from the previous time samplesare computed and summed and stored in the register 790. When a new timesample is introduced, the normalization value S is computed bydiscarding the multiplication term of the obsolete sample which is the(N+1)th previous sample (multiplier 782) and adding the multiplicationterm of the new sample (multiplier 784) to the stored normalizationvalue in register 790. As a result, only two multiplications areperformed at each time sample—one for the new sample and one for theobsolete sample. The product based on the new sample is added to thestored sum and the product based on the obsolete sample is subtractedfrom the stored sum at summer 786 to generate the updated normalizationvalue S.

The registers for storing values of R and S are updated at sample rate.However, the calculation of tap-specific gain control metricg_(i)[τ]=(R[τ]/S)², τ=0, 1, L, N_(tap)−1, and the corresponding gainadjustment are usually performed in a much slower pace than the samplerate.

5. Gain Adjustment Stepping Control

FIG. 13 above illustrates a repeater using a gain control metriccalculator for monitoring and computing a gain control metric. The gaincontrol metric then drives the gain control algorithm which controls theoutput gain of the repeater based at least in part on the gain controlmetric. According to embodiments of the present invention, the gainadjustment step size of the variable gain stage is a function of atleast the loop gain of the repeater as measured by the gain controlmetric. In some embodiments, the gain adjustment step size of thevariable gain stage is a function of the loop gain and the variable gainof the repeater. More specifically, in one embodiment, the loop gain ofthe repeater is divided into different operating zones and the gainadjustment step size is zone dependent. Finally, in another embodiment,the gain adjustment step size is a function of the gain control state,including the boot-up state and the steady state of the repeateroperation.

FIG. 18 is a diagram illustrating the gain adjustment control zonesaccording to one embodiment of the present invention. Referring to FIG.18, in steady state operation, the loop gain is in the cool zone (−20 dBto −25 dB) and the gain value does not vary very much due to the steadystate operation. Thus, a gain adjustment control curve 802 having asmall slope is used to provide a small gain adjustment step size. In theboot-up state, if the loop gain is in the target zone (−15 dB to −20 dB)or in the cool zone indicating stable operation, the rate of gainincrease is a decreasing function of the total repeater output gain(Δ⁺dB/dB). That is, the gain adjustment step size is a decreasingfunction of the repeater gain. The higher the repeater output gain, thesmaller the gain adjustment step size or a small gain adjustment slopeis used. This is because the higher the repeater output gain, the closerto instability the repeater is. Therefore, a smaller gain adjustmentstep size for increasing the gain is used when the gain is high. Curves804 bond the area where the family of gain adjustment control curvesresides. As the repeater output gain increases, gain adjustment curveswith smaller slope is used.

In the hot zone (−10 dB to −15 dB) or the alert zone (−5 dB to −10 dB),the gain of the repeater decreases according to a selected one of gainadjustment control curves 806. Gain adjustment control curves 806(curves 1-4) in the alert zone and hot zone illustrate the differentgain adjustment stepping control that can be used to decrease therepeater gain when the gain value is in the hot or alert zone. Morespecifically, the rate of gain decrease is set as an increasing functionof the total repeater output gain (Δ⁺ dB/dB). That is, the larger therepeater output gain, the closer to instability and thus a steep gainadjustment slope is used to decrease the gain rapidly in the hot oralert gain adjustment control zone. For instance, at low repeater gainlevels, curve 1 having a piecewise linear slope is used where the gainis not adjusted when the loop gain is in the hot zone but when the loopgain increases to the alert zone, then the repeater gain is decreasedaccording to the slope of curve 1. In one embodiment, curve 1 has aslope of −1 dB/dB. A non-linear stepping control can be used as shown bycurve 2. When the repeater output gain is large, a steep slope for thegain adjustment step size is used as shown by curve 4 to rapidlydecrease the gain.

Finally, if the loop gain is in the guard zone (0 dB to −5 dB), therepeater gain is decreased very rapidly (gain adjustment control curve808) as the loop gain approaches instability. In the present embodiment,the gain adjustment control curve 808 thus has a steep linear slope. Inother embodiments, the gain adjustment control curve 808 can have linearor non-linear slope and an appropriate slope value to provide thedesired gain adjustment stepping control.

In one embodiment, the multi-metric gain control method described aboveis used to monitor the gain of the repeater to determine which of thegain adjustment control zones the repeater is operating in. Morespecifically, in one embodiment, a slow gain control metric is used tomonitor the loop gain under −5 dB for improved accuracy and a fast gaincontrol metric is used to monitor the loop gain above −5 dB for fastgain control response, as shown in FIG. 18. In other embodiments, theslow and fast gain control metric can be applied to other gainadjustment control zones, such as −15 dB.

FIG. 19 is a flowchart illustrating the gain adjustment stepping controlmethod as applied to the repeater of FIG. 6 implementing multiple metricgain control according to one embodiment of the present invention.Referring to FIG. 19, gain adjustment stepping control method 810 startsby setting a fast counter to 0 (step 812) and also setting a slowcounter to 0 (step 814). Then, method 810 waits for a new sample (step816). The sample buffer is updated (step 818) and the slow and fast gaincontrol metrics are also updated based on the new sample (step 820).Then the fast and slow counters are incremented (step 822). If the fastcounter does not equal to the Fast Delay value, (step 824), then method810 returns to step 816 to wait for another new sample. If the fastcounter is equal to the fast delay (step 824), then the fast gaincontrol metric has finished integration. Method 810 proceeds todetermine if the loop gain is in the guard zone of 0 to −5 dB (step 826)since the fast gain control metric is used to monitor the loop gain inthe guard zone.

If the loop gain is in the guard zone, then method 810 reduces the gainof the repeater using curve 808 (step 828) and return to step 812 wherethe fast counter is reset. If the loop gain is not in the guard zone(step 826), then method 810 moves to determine if the slow gain controlmetric has finished integration by determining if the slow counter isequal to the slow delay (step 830). If the slow counter does not equalto the slow delay, then method 810 returns to step 816 for receivinganother new sample. If the slow counter is equal to the slow delay, theslow gain control metric has completed integrated and method 810determines the zone the loop gain is operating in (step 834). That is,method 810 determines if the loop gain is operating in the cool zone,the target zone, the hot zone or the alert zone. The repeater gain isthen adjusted based on the operating zone and the current repeateroutput gain specific functions (step 836). Method 810 returns to step814 where the slow counter is reset and the method continues.

In this manner, the gain adjustment stepping control method 810 updatesthe slow and fast gain control metrics, uses the fast metric todetermine if the loop gain is in the guard zone and reduces the gain ifthe loop gain is in the guard zone. The gain control method further usesthe slow metric to determine if the loop gain is in the other zones(cool, target, hot, and alert) and update the gain adjustment accordingto the zone and the current output gain specific functions. Fine tunegain control is thus realized.

6. Gain Control Metric Bias Removal

Gain control is essential for the safe boot up and stable operation of arepeater. Gain control metric is a quantity that a gain controlalgorithm uses for repeater gain adjustment. For instance, FIG. 13illustrates a repeater using a gain control metric calculator formonitoring and computing a gain control metric and a gain controlalgorithm for controlling the repeater gain in response to the gaincontrol metric. However, the gain control metric estimate is highlybiased due to estimation noise. The presence of bias prevent gaincontrol algorithm from accurately estimating the stability of therepeater.

As discussed above, the tap-specific gain control metric is given as:

${g_{i}(\tau)} = {{\frac{\sum\limits_{n = 0}^{N - 1}{{p^{*}\left\lbrack {n + i} \right\rbrack}{p\left\lbrack {n + i + N_{delay} + \tau} \right\rbrack}}}{\sum\limits_{n = 0}^{N - 1}{{p\left\lbrack {n + i} \right\rbrack}}^{2}}}^{2} = {{\eta_{i}(\tau)}}^{2}}$

The gain metric bias results from the variance of the normalizedcorrelation signal η. In one embodiment, the bias is expressed as:

Bias{g _(i)(τ)}=Var{η_(i)(t)}

In particular, the variance results because of the squaring of thecomplex number η to obtain a real number. The gain control metricbecomes highly biased as gain control metric estimation noise increases.More specifically, as discussed above, the gain control metric is ameasure of the correlation between the transmitted signal and thereceived signal. A large correlation indicates a large amount of leakageand less stability. The gain control algorithm will respond to the gaincontrol metric and lower the gain. A small correlation indicates a smallfeedback signal and increased stability. The gain control algorithm willrespond to the gain control metric and increase the gain. However, evenwhen there is no correlation, i.e., even when there is no feedbacksignal detected in the received signal, the squaring of the complexvariable η will still give some value in real number. A bias in the gaincontrol metric thus results.

According to one embodiment of the present invention and illustrated inFIG. 9, a bias estimator 590 is incorporated in the gain metriccalculator to estimate the bias in the gain control metric. The varianceof the gain control metric is estimated at each delay lag τ over time asa value δ as follows:

$\delta = {\sum\limits_{\tau = 1}^{N_{tap}}{\left\{ {\underset{i}{var}\left\{ {\eta_{i}(\tau)} \right\}} \right\}.}}$

FIG. 20 illustrates the computation of the metric variance over delaylags and over time according to one embodiment of the present invention.The variance is computed for each delay lag and at each time sample,then the variance is added up to estimate the variance across the delaylags.

Once the bias value δ is computed, the bias δ is subtracted from thegain control metric as follows:

$g_{i} = {{\sum\limits_{\tau = 1}^{N_{tap}}\left\{ {g_{i}(\tau)} \right\}} - {\delta.}}$

When the gain metric calculator uses multiple gain control metrics, thebias δ is subtracted from each of the gain control metrics.

7. Gain Control Metric Pruning

During the operation of a same frequency repeater, it is desirable tomaintain stability in the repeater in the presence of large signaldynamics. Digital gain control may be used to maintain stability in arepeater. However, any gain control algorithm needs to be able tomeasure the stability margin, represented by the loop gain, so it candetermine the appropriate gain to maintain stability. If there are largescale power swings in the remote signal, typical means of measuring theloop gain are susceptible to inaccuracy, and gain control may notfunction as desired.

According to one embodiment of the present invention, a gain controlalgorithm that is robust in the presence of large scale signal dynamicsof the remote signal is described. More specifically, the gain controlalgorithm implements gain control metric pruning to discard gain controlmetrics, or at least parts of the gain control metrics, associated withlarge change in the signal dynamics of the remote signal. In thismanner, the overall gain control metric values will not be corrupted dueto abrupt changes in the power level of the receive signal and robustgain control is realized for achieving stability.

In one embodiment, the stability margin, as represented by the loopgain, in a repeater is determined by computing a correlation of thetransmitted signal with the feedback signal. This quantity is normalizedby the power of the transmitted signal in order to obtain an unbiasedestimate of the loop gain. The expression for the loop gain estimationor the gain control metric is described above and repeated here. Thegain control metric is characterized as a square of a correlation termR, in the numerator divided by a normalization term S, in thedenominator, given as:

g[τ]=(R[τ]/S)² τ=0,1,L,N_(tap)−1.

However, if the power in the remote signal suddenly increases by a largeamount, the correlation output will have a sudden jump, and at a latertime, determined by the delay through the repeater, the transmittedsignal will have a jump in energy. Because of this delay, for a shorttime the correlation energy normalized by the transmitted energy will beextremely high, indicating a large normalized correlation which in factdoes not exist. As a result, a conventional gain control algorithm willkeep the repeater gain abnormally low in the presence of large jumps inremote signal power. In other words, the gain control metricmeasurements become corrupted in the presence of large swings in thesignal power of the remote signal and the gain control algorithm cannotfunction properly. The same corruption of the gain metric measurementswould be observed for other forms of gain control metrics as typicalgain control metrics include a correlation term.

FIG. 21 is a flowchart illustrating the gain control metric pruningmethod implemented in a gain control algorithm according to oneembodiment of the present invention. The gain control metric pruningmethod of the present invention can be implemented in an echocancellation repeater, such as the repeater shown in FIG. 13, or themethod can be implemented in a repeater without echo cancellation, suchas the repeater shown in FIG. 15. The gain control metric pruning methodcan be implemented in the gain metric calculator 660, 760 of therepeaters in FIGS. 13 and 15.

Referring to FIG. 21, a gain control metric pruning method 900implemented in a gain control algorithm operates to disregard at least aportion of the gain control metric measurements for a short durationafter a large jump in the power signal of the remote signal has beendetected. Method 900 starts by receiving the incoming receive signal ata repeater which includes the remote signal (step 902). As describedabove, the receive signal is the sum of the remote signal and thefeedback signal. A signal in the feedback loop of the repeater is usedas the gain control input signal where samples of the gain control inputsignal are used to compute the gain control metric. The gain controlinput signal can be taken from any point in the repeater feedback loop,including before echo cancellation or after echo cancellation, or anypoint in the repeater feedback loop without echo cancellation. Then,swings in the power level of the gain control input signal are detectedto determine if there are large power swings in the signal (step 904).The swings in the power level of the gain control input signal can bedetected directly from the gain control input signal or it can bedetected indirectly through measurements of other signals having a powerlevel response corresponding to the gain control input signal. In oneembodiment, power swings are detected by measuring the power swing inthe receive signal. In other embodiments, swings in the gain controlinput signal is detected by using an FIR (finite impulse response) orIIR (infinite impulse response) filter. When no large power swings aredetected in the gain control input signal (step 906), method 900 returnsto step 902 to continue to receive the incoming receive signal. However,when a large power swing, such as a power swing of 9 dB or greater, isdetected (step 906), gain control metric measurements, or at least aportion of each gain control metric measurement, are discarded for atime period of T (step 908). In one embodiment, the correlationmeasurements R_(i) of the gain control metric measurements are discardedfor the time period of T. The duration T over which portions of gaincontrol metric measurements are disregarded is short enough (on theorder of 10 μs) such that the ability to detect possible instabilitiesin normal repeater operation is not impeded. Method 900 than returns tostep 902 to continue to receive incoming receive signal.

FIG. 22 is a flowchart illustrating the gain control metric pruningmethod implemented in a gain control algorithm according to an alternateembodiment of the present invention. Method 950 in FIG. 22 is similar tomethod 900 in FIG. 21 and like elements are given like referencenumerals to simplify the discussion. Referring to FIG. 22, in gaincontrol metric pruning method 950, the gain control metric pruningmethod operates to discard samples of the gain control input signal usedfor the gain control metric computation for a short duration T after alarge jump in remote signal power has been detected (step 958). In thismanner, samples of the gain control input signal associated with swingsin the remote signal power are discarded before the samples corrupt thegain control metric measurements.

The advantage of using gain control metric pruning in accordance is thata gain control algorithm for an on-frequency repeater can run robustlyin the presence of large scale signal dynamics of the remote signal. Itis crucial to implement a gain control algorithm to ensure stability. Ifgain control metric pruning were not implemented, then gain controlwould not allow the gain to ramp up in the presence of large swings insignal power, and the repeater maybe completely dysfunctional under suchcircumstances. However, when the gain control metric pruning method ofthe present invention is implemented, the gain control algorithm is ableto control the gain of the repeater in the presence of large swings inthe signal power of the remote signal to allow the repeater to respondeffectively to changing conditions of the remote signal

Those skilled in the art will understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example: data, information, signals, bits, symbols,chips, instructions, and commands may be referenced throughout the abovedescription. These may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

In one or more exemplary embodiments, the functions and processesdescribed may be implemented in hardware, software, firmware, or anycombination thereof. If implemented in software, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by acomputer. By way of example, and not limitation, such computer-readablemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium that can be used to carry or store desired program code inthe form of instructions or data structures and that can be accessed bya computer. Disk and disc, as used herein, includes compact disc (CD),laser disc, optical disc, digital versatile disc (DVD), floppy disk andblu-ray disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers. Combinations of the aboveshould also be included within the scope of computer-readable media. Theterm “control logic” used herein applies to software (in whichfunctionality is implemented by instructions stored on amachine-readable medium to be executed using a processor), hardware (inwhich functionality is implemented using circuitry (such as logicgates), where the circuitry is configured to provide particular outputfor particular input, and firmware (in which functionality isimplemented using re-programmable circuitry), and also applies tocombinations of one or more of software, hardware, and firmware.

For a firmware and/or software implementation, the methodologies may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. Any machine readable mediumtangibly embodying instructions may be used in implementing themethodologies described herein. For example, software codes may bestored in a memory, for example the memory of mobile station or arepeater, and executed by a processor, for example the microprocessor ofmodem. Memory may be implemented within the processor or external to theprocessor. As used herein the term “memory” refers to any type of longterm, short term, volatile, nonvolatile, or other memory and is not tobe limited to any particular type of memory or number of memories, ortype of media upon which memory is stored.

Also, computer instructions/code may be transmitted via signals overphysical transmission media from a transmitter to a receiver. Forexample, if the software is transmitted from a website, server, or otherremote source using a coaxial cable, fiber Optic cable, twisted pair,digital subscriber line (DSL), or physical components of wirelesstechnologies such as infrared, radio, and microwave. Combinations of theabove should also be included within the scope of physical transmissionmedia.

Moreover, the previous description of the disclosed implementations isprovided to enable any person skilled in the art to make or use thepresent invention. Various modifications to these implementations willbe readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other implementationswithout departing from the spirit or scope of the invention. Thus, thepresent invention is not intended to be limited to the features shownherein but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

1. A method for computing a gain control metric used in controlling gainin a wireless repeater, the wireless repeater receiving an input signalat a receiving antenna and transmitting an output signal on thetransmitting antenna, the input signal being a sum of a remote signal tobe repeated and a feedback signal resulting from a feedback channelbetween the receiving antenna and a transmitting antenna, the feedbackchannel thus forming a feedback loop between the transmitting antennaand the receiving antenna, the method comprising: providing atap-specific gain control metric indicative of a loop gain of therepeater, the tap-specific gain control metric at a given channel tapbeing computed from a gain control input signal and being a square of acorrelation term divided by a normalization term, the correlation termand the normalization term having an integration length of N, thecorrelation term being indicative of a correlation of the gain controlinput signal and a delayed version of the gain control input signal andthe normalization term being indicative of a power of the gain controlinput signal that is not delayed, the gain control input signal beingtaken from any point in the feedback loop of the repeater; providing afirst register for storing a stored correlation value, the storedcorrelation value being the correlation term computed over a previous Nsamples; providing a second register for storing a stored normalizationvalue, the stored normalization value being the normalization termcomputed over a previous N samples; receiving a new sample of the gaincontrol input signal; computing an updated correlation value byperforming a first multiplication using an obsolete sample of the gaincontrol input signal, the obsolete sample being the (N+1)th previoussample, and a second multiplication using the new sample of the gaincontrol input signal, the updated correlation value being computed byadding the product from the second multiplication and subtracting theproduct from the first multiplication; computing an updatednormalization value by performing a third multiplication using anobsolete sample of the gain control input signal, the obsolete samplebeing the (N+1)th previous sample, and a fourth multiplication using thenew sample of the gain control input signal, the updated normalizationvalue being computed by adding the product from the fourthmultiplication and subtracting the product from the thirdmultiplication; and computing the tap-specific gain control metric usingthe updated correlation value and the updated normalization value. 2.The method of claim 1, further comprising: computing the tap-specificgain control metric over each channel tap using the updated correlationvalue and the updated normalization value; and computing a gain controlmetric based on a combination of the computed tap-specific gain controlmetric over all channel taps.
 3. The method of claim 2, whereincomputing a gain control metric based on a combination of the computedtap-specific gain control metric over all channel taps comprisescomputing a gain control metric based on a linear combination of thecomputed tap-specific gain control metrics over all channel taps.
 4. Themethod of claim 2, wherein computing a gain control metric based on acombination of the computed tap-specific gain control metric over allchannel taps comprises computing a gain control metric based on anon-linear combination of the computed tap-specific gain control metricsover all channel taps.
 5. The method of claim 1, wherein thetap-specific gain control metric at a given channel tap τ isg_(i)[τ]=(R[τ]/S)², where R[τ] is the correlation term and S is thenormalization term.
 6. A wireless repeater having a receiving antennafor receiving an input signal and a transmitting antenna fortransmitting an output signal, the input signal being a sum of a remotesignal to be repeated and a feedback signal resulting from a feedbackchannel between the receiving antenna and the transmitting antenna, thefeedback channel thus forming a feedback loop between the transmittingantenna and the receiving antenna, the repeater comprising: a gaincontrol block configured to control a gain of the repeater, the gaincontrol block comprising a gain metric calculator configured to receivea gain control input signal and to compute a tap-specific gain controlmetric indicative of a loop gain of the repeater, the tap-specific gaincontrol metric at a given channel tap being a square of a correlationterm divided by a normalization term, the correlation term and thenormalization term having an integration length of N, the correlationterm being indicative of a correlation of the gain control input signaland a delayed version of the gain control input signal and thenormalization term being indicative of a power of the gain control inputsignal that is not delayed, the gain metric calculator comprising: afirst register configured to store a stored correlation value, thestored correlation value being the correlation term computed over aprevious N samples of the gain control input signal; and a secondregister configured to store a stored normalization value, the storednormalization value being the normalization term computed over aprevious N samples of the gain control input signal, wherein the gainmetric calculator block is configured to receive a new sample of thegain control input signal and the gain metric calculator is configuredto compute an updated correlation value by performing a firstmultiplication using an obsolete sample of the gain control inputsignal, the obsolete sample being the (N+1)th previous sample, and asecond multiplication using the new sample of the gain control inputsignal, the updated correlation value being computed by adding theproduct from the second multiplication and subtracting the product fromthe first multiplication; and wherein the gain metric calculator isconfigured to compute an updated normalization value by performing athird multiplication using an obsolete sample of the gain control inputsignal, the obsolete sample being the (N+1)th previous sample, and afourth multiplication using the new sample of the gain control inputsignal, the updated normalization value being computed by adding theproduct from the fourth multiplication and subtracting the product fromthe third multiplication, the gain metric calculator being configured tocompute the tap-specific gain control metric using the updatedcorrelation value and the updated normalization value.
 7. The wirelessrepeater of claim 6, wherein the gain metric calculator is configured tocompute the tap-specific gain control metric over each channel tap usingthe updated correlation value and the updated normalization value and again control metric is computed based on a combination of the computedtap-specific gain control metrics over all channel taps.
 8. The wirelessrepeater of claim 7, wherein the gain control metric is computed basedon a linear combination of the computed tap-specific gain controlmetrics over all channel taps.
 9. The wireless repeater of claim 7,wherein the gain control metric is computed based on a non-linearcombination of the computed tap-specific gain control metrics over allchannel taps.
 10. The wireless repeater of claim 6, wherein thetap-specific gain control metric at a given channel tap τ isg_(i)[τ]=(R[τ]/S)², where R[τ] is the correlation term and S is thenormalization term.
 11. The wireless repeater of claim 6, wherein thegain metric calculator is configured to take the gain control inputsignal from any point in the feedback loop of the repeater.
 12. Awireless repeater having a receiving antenna for receiving an inputsignal and a transmitting antenna for transmitting an output signal, theinput signal being a sum of a remote signal to be repeated and afeedback signal resulting from a feedback channel between the receivingantenna and the transmitting antenna, the feedback channel thus forminga feedback loop between the transmitting antenna and the receivingantenna, the repeater comprising: means for controlling a gain of therepeater, the means comprising a first means for receiving a gaincontrol input signal and computing a tap-specific gain control metricindicative of a loop gain of the repeater, the tap-specific gain controlmetric at a given channel tap being a square of a correlation termdivided by a normalization term, the correlation term and thenormalization term having an integration length of N, the correlationterm being indicative of a correlation of the gain control input signaland a delayed version of the gain control input signal and thenormalization term being indicative of a power of the gain control inputsignal that is not delayed, the first means comprising: a second meansfor storing a stored correlation value, the stored correlation valuebeing the correlation term computed over a previous N samples of thegain control input signal; and a third means for storing a storednormalization value, the stored normalization value being thenormalization term computed over a previous N samples of the gaincontrol input signal, wherein the first means is configured forreceiving a new sample of the gain control input signal and computing anupdated correlation value by performing a first multiplication using anobsolete sample of the gain control input signal, the obsolete samplebeing the (N+1)th previous sample, and a second multiplication using thenew sample of the gain control input signal, the updated correlationvalue being computed by adding the product from the secondmultiplication and subtracting the product from the firstmultiplication; and wherein the first means is configured for computingan updated normalization value by performing a third multiplicationusing an obsolete sample of the gain control input signal, the obsoletesample being the (N+1)th previous sample, and a fourth multiplicationusing the new sample of the gain control input signal, the updatednormalization value being computed by adding the product from the fourthmultiplication and subtracting the product from the thirdmultiplication, the gain metric calculator being configured to computethe tap-specific gain control metric using the updated correlation valueand the updated normalization value.